Depends on what interest your wife has:

a. if she's a tech savvy person - might give her one of the latest trend gadgets... if i were her, i'd be happy to have ipad and iphone

b. is she loves shopping - then just give her money tell her to spend to whichever things she wanted to buy (aw~ love it too)

c. if she's into romantic surprises,, a romantic dinner with special event will be helpful (Example: http://www.youtube.com/watch?v=--A5IIhqXOc)
My friend did this, he asked people to write a note and he took some photos of it then made video compilation...he played it on their TV when his GF went home + a surprise candle lit dinner) So sweet~~

d. Or just ask her what she wants... hope that i helped you^^